JP2007508702A - Surface emitting semiconductor laser with structured waveguide. - Google Patents

Abstract

The laser has an active zone with a pn junction enclosed by a first n-doped semiconducting layer and at least one p-doped semiconducting layer, a tunnel contact layer on the p-side of the active zone with an aperture with an aperture diameter and depth and covered by an n-doped current carrying layer (7) with an elevation in the aperture region. A structured layer (8) about the lateral area of the elevation has a thickness selected so its optical thickness is at least equal to that of the current carrying layer near the elevation depth.

Description

  The present invention relates to a surface emitting semiconductor laser having a structured waveguide.

  A vertical cavity surface emitting laser (VCSEL) is a semiconductor laser in which light emission occurs perpendicular to the surface of a semiconductor chip. Vertical cavity surface emitting laser diodes have low power consumption, can directly inspect laser diodes on the wafer, can simply couple a longitudinal single-mode spectrum to an optical fiber, surface in a two-dimensional matrix There are several advantages over conventional surface emitting laser diodes, such as the ability to connect emitting laser diodes.

  In the field of communication technology using optical fibers, there is a need for VCSELs in the wavelength range of about 1.3 μm to 2 μm, especially around wavelengths of 1.31 μm or 1.55 μm, due to wavelength dependent dispersion and absorption. This type of long wavelength laser diode has heretofore been manufactured from an InP-based compound semiconductor. GaAs-based VCSELs are suitable for short wavelength ranges below 1.3 μm.

  The previous solutions can be summarized as follows.

  The characteristics of the lateral beam of the laser diode are significantly affected by the shape of the corresponding waveguide. In the case of a GaAs-based VCSEL having an emission wavelength of less than about 1.3 μm, the waveguide is fabricated from a selectively oxidized Al (Ga) As layer (“electrically pumped 10 Gbit / s MOVPE with GaInNAs active region). Growing Monolithic 1.3 μm VCSEL ”Electronics Letter, Vol. 38, No. 7 (March 28, 2002), pages 322-324).

  In particular, the best results are seen for InP-based BTJ (buried tunnel junctions) for long wavelength VCSELs in the wavelength range above 1.3 μm in terms of power, operating temperature, single mode power and modulation bandwidth. ) VCSEL.

The manufacture and structure of the buried tunnel contact portion will be described using an example with reference to FIG. Molecular beam epitaxy (MBE) is used to create highly doped p ⁺ / n ⁺ layer pairs 101, 102 with a low band gap. The tunnel contact portion 103 itself is formed between these two layers. The circular or elliptical region is substantially formed from the n ⁺ doped layer 102, the tunnel contact 103, and part or all of the p ⁺ doped layer 101 and is formed by reactive ion etching (RIE). ing. In the second epitaxial cycle, this region is overgrown with n-doped InP (layer 104) so that the tunnel contact is “buried”. The contact region between the overgrown layer 104 and the p ⁺ doping layer 101 acts as a barrier layer when a voltage is applied. The current generally flows through the tunnel contact with a resistance of 3 × 10 ⁻⁶ Ωcm ² . Current flow is thus limited to the active region of the active zone 108. The amount of heat generated is small because current flows from the high resistance p-doped layer to the low resistance n-doped layer.

  Overgrowth of the tunnel contact results in a slight variation in the thickness of the overlying layer, as shown in FIG. This adversely affects the lateral waveguiding. The formation of higher transverse modes is facilitated especially at relatively large openings. Thus, a small aperture and correspondingly low laser power can be used for single mode operation, particularly required in fiber optic communication technology.

  The overall structure of the InP-based VCSEL will now be described using an example with reference to FIG.

In this structure, the buried tunnel junction (BTJ) is provided in the periphery in another manner, so that the active zone 106 has a tunnel contact (diameter) between the p ⁺ doping layer 101 and the n ⁺ doping layer 102. D _BTJ ). Laser radiation is directed in the direction indicated by arrow 116. The active zone 106 is surrounded by a p-doped layer 105 (eg, InAlAs) and an n-doped layer 108 (eg, InAlAs). The tip side mirror 109 on the active zone 106 consists of an epitaxial DBR (distributed reflection laser) containing a pair of about 35 InGa (Al) As / InAlAs layers, resulting in a reflectivity of about 99.4%. The rear end side mirror 112 is composed of a stack of dielectric layers as DBR and is finished with a gold layer, so that a reflectivity of approximately 99.75% is generated. The insulating layer 113 is used for lateral insulation. A further p-side contact layer 111 structured in a ring is provided between the layer 104 and the contact layer 114. FIG. 2 shows how the structure of the overgrown tunnel contact propagates (in this case downwards) to further layers.

  The combination of the dielectric mirror 112, the integrated contact layer 114, and the heat sink 115 results in greatly improved thermal conductivity compared to an epitaxial multilayer structure. The current is injected via the contact layer 114 or via the integrated heat sink 115 and the n-side contact point 110. Reference is made to the following cited references for further details on the manufacture and characteristics of such VCSEL types.

  The VCSEL having the structure shown in FIG. 2 is a publication “Highly Efficient Low Threshold Index Guide 1.5 μm Long Wavelength Vertical Cavity Surface Emitting Laser”, Applied Physics Letter, Vol. 16 (April 17, 2000), 2179-2181 pages. The same type of VCSEL with an output power up to 7 mW (20 ° C., CW) is described as “1.55 μm vertical cavity surface emitting laser diode with high output power and high operating temperature”, Electronics Letter, Vol. 37, No. . 21 (October 11, 2001) 1295-1296. Published material “90 ° C continuous wave operation of 1.83 μm vertical cavity surface emitting laser” IEEE Photonics Technical Letter, Vol. 11 (November 2000) pages 1435 to 1437 relate to a 1.83 μm InGaAlAs-InP VCSEL. “High-speed data transmission with 1.55 μm vertical cavity surface emitting laser”, Post Deadline Paper, 28th European Conference on Optical Communications (September 8-12, 2002), modulation frequencies up to 10 Gbit / s Discusses the use of BTJ-VCSEL for error-free data transmission in Finally, VCSELs having a radiation wavelength of 2.01 μm (CW) are described in “Electropumped room temperature CW-VCSEL with radiation wavelength of 2 μm” Electronics Letter, Vol. 1 (January 9, 2003), pages 57-58.

  In contrast to GaAS-based VCSELs with emission wavelengths less than 1.3 μm, the lateral oxidation method cannot be used for the BTJ-VCSEL discussed. This is because the material used has a very small amount of aluminum component, and other conceivable materials such as AlAsSb have not provided a sufficient quality oxide layer so far. In the above-described BTJ-VCSEL, the lateral waveguide effect resulting from the manufacturing process is performed as a variation in the lateral length of the resonator. Instead, selectively etched away layers (“1.55 μm ImP lattice matched VCSEL with AlGaAsSb—AlAsSb DBR”, IEEE Journal on Quantum Electron Selection Topic, Vol. 7, No. 2 (2001 (March / April), see pages 224-230), proton injection ("modified DBR and tunnel junction injection: CW RT monolithic long wavelength VCSEL", IEEE Journal on Quantum Electron Selection Topic, Vol. 5, No. 3 (See May / June 1999, pages 520-529), or selectively oxidized modified AlAs layers ("1.5-1.6 μm VCSEL for metro WDM applications" relating to indium phosphide and related materials Minutes of 2001 International Conference, 13th IPRM (May 14-18, 2001), Japan Kuniara) is used, for example, in other long wavelength VCSEL designs.

  The object of the present invention is therefore to replace the index guide, which is a powerful and preferred multimode operation in the case of BTJ-VCSEL, with a weaker index guide or gain guide, and optionally weaken the higher transverse mode. It is in. The adjustment of the transverse mode characteristics allows single mode operation even with large apertures having higher single mode power than conventional BTJ-VCSEL.

  This object is achieved by a surface emitting semiconductor laser claimed according to the invention. Further configurations will become apparent from the respective dependent claims and the following description.

  The present invention comprises an active zone having a pn junction and surrounded by a first n-doped semiconductor layer and at least one p-doped semiconductor layer, and a tunnel contact layer on the p side of the active zone. A surface emitting semiconductor laser, wherein the tunnel contact layer comprises an opening having an opening diameter and an opening depth and is covered with an n-doping current carrying layer, and an adjacent current carrying layer is formed in the region of the opening, A ridge having a ridge diameter and a ridge depth; and a structured layer is provided on the current carrying layer at least around a side region of the ridge, the thickness of which is optical of the structured layer. The thickness is selected to be at least equal to the optical thickness of the current carrying layer in the region of the ridge depth.

  In order to facilitate understanding of the present invention, the conditions in the known structure of a typical surface emitting semiconductor laser will first be described as detailed in the introduction of the description. Therefore, although described with reference to FIG. 3, here, conditions in a known structure of a general surface emitting semiconductor laser are schematically illustrated without being enlarged or reduced. The figure shows the boundary region between the current carrying layer (7) and the n-doped contact layer 8, through which the current is supplied entirely and is highly n-doped with a thickness d3. Conveniently grows from InGaAs to layer 7. A ridge formed by overgrowth of the tunnel contact and having a layer 7 of thickness d2 (= depth of the ridge) is shown as 15. The contact layer 8 is conventionally provided in an epitaxial step and is selectively etched away in the region of the ridges 15. The structured contact layer 8 is generally provided with a thickness d3 of 50 to 100 nm, resulting in a low contact resistance, with a distance of a few micrometers (typically 4 to 5 μm) from the tunnel contact ridge 15 at its inner edge. is there. In the structure shown in the drawing, the length of the resonator is thicker by d2 at the center than the region outside the raised portion 15. The effective refractive index is higher in the center (typically by 1%) than in the outer region, making it a strong index guide. This contributes to the formation of higher modes, especially with large openings.

  In order to weaken the index guide, the present invention therefore proposes to add a structured layer at least around the lateral region of the ridge 15. The optical thickness is at least equal to the optical thickness of the current carrying layer 7 in the region of the raised portion 15, that is, the optical thickness of the raised portion 15 having the thickness d2. The structured layer according to the invention therefore corrects for the difference in refractive index between the center of the ridge 15 and the outer region, resulting in a significantly weakened index guide.

  Therefore, the structured layer according to the present invention needs to be adjacent to the ridge 15 or within a specified maximum distance from the ridge. It has been found that this maximum distance should be 2 μm or less, preferably 1 μm or less. This maximum distance therefore corresponds to 40-50%, preferably 20-25% of the typical distance of the (optional) contact layer 8 from the outer edge of the ridge 15 to date (see FIG. 3). reference).

  It has proved effective if the structured layer according to the invention is an n-doped contact layer.

  This type of contact layer is self-evident from the prior art (see FIG. 3). In this example, the thickness of the contact layer is thus made such that its optical thickness is equal to, for example, the optical thickness of the current carrying layer in the region of depth d2 of the raised portion 15 (see FIG. 3). There, the contact layer should not be larger than 1-2 μm from the ridge in order to be fully affected by the optical field.

  In another embodiment, a structured layer according to the present invention is provided independently of the optional contact layer. The material from which the aforementioned structured layer is made is freely chosen, and the layer is conveniently directly adjacent to the ridge of the current carrying layer, where the same principle is structured at any distance from the ridge Applied to the thickness of the structured layer with respect to the contact layer as a layer. Free material selection can be used to more intensely suppress higher modes at the edge of the aperture, especially because of the strong field coverage in the outer region, thus avoiding oscillation of these modes. . Materials having a significant absorption action at each wavelength are generally suitable for this purpose. Amorphous silicon is particularly suitable for wavelengths between 1.3 μm and 1.55 μm. Titanium is suitable, for example, for the entire conventional wavelength range.

  In the embodiments described above, a contact layer surrounding the structured layer according to the invention can also be provided. The geometry of the contact layer can be chosen virtually freely since the waveguiding is already compensated by the structured layer according to the invention.

  It has been found that the present invention also exhibits an anti-guiding effect and eliminates high modes since the index guide can be converted as well as weakened. The (optional) thickness of the structured layer according to the invention is therefore selected to be considerably thicker than the depth of the ridge due to the tunnel contact. Thus, ridges are created in the outer region, where high mode removal is even more effective when the structured layer has an absorbing action. In this embodiment with anti-guiding action, the structured layer used is again an n-doped contact layer or combination of layers, the material of which can be freely selected as a structured layer and an optional additional contact layer.

  The invention will now be described in detail with reference to various embodiments illustrated in the drawings.

  1-3, reference numerals are appended to the foregoing portion of the description.

FIG. 4 shows an example of a structure formed during the manufacture of a surface emitting semiconductor laser as will be described. Starting from the InP substrate 1, an n-doping epitaxial Bragg mirror 2, an n-doping confinement layer 3, an active zone 4 and a p-doping confinement layer 5 are successively added in a first epitaxial growth process. This structure is completed by the growth of the tunnel contact layer 6 for each case of highly p ⁺ and n ⁺ doped InGaAs layers, for example. The opening can be freely chosen in its dimensions and extends to layer 5 or ends within the p-doping portion of layer 6 and is formed in a later lithographic and etching process. A typical etching depth is 20 nm in this case.

  In the second epitaxial step, an upper n-doping current supply layer 7 advantageously made of InP and an optional n-contact layer 8 conveniently made of highly n-doped InGaAs are grown to a thickness d3. In this second epitaxial step, the lateral semi-axis ratio is based on process parameters or epitaxial methods (eg MBE (molecular beam epitaxy), CBE (chemical beam epitaxy) or MOVPE (metal organic vapor phase epitaxy). The modification, for example, results in an elliptical opening in the previously circular tunnel contact.

  The result is shown in FIG. 4, for example, a round opening having a diameter w1 is shaped as a lithographic forming opening having an etching depth of d1, and after overgrowth, becomes a diameter w2 having a height of d2. The values w2 and d2 generally correspond to the starting data w1 and d1.

  The term “diameter” or “radius” used in the description is not meant to be limited to the shape of the circular opening, as the invention is applicable to openings other than circular. Angular, elliptical, or any other shape is possible and the present invention can be easily modified to such a shape.

  A structure starting from the structure shown in FIG. 4 and obtained after selective etching of the contact layer 8 is shown here as a first embodiment in FIG. In this case, the contact layer 8 acts as a structured layer according to the invention. The thickness d3 of the contact layer is selected, for example, so as to completely correct the waveguiding effect so that its optical thickness corresponds to the optical thickness of the region of the etching depth d2 of the layer 7. An arrangement approximately parallel to the plane is obtained there. In a specific region, the etching diameter w3 can be adjusted almost arbitrarily as desired. However, to sufficiently affect the optical field, the radius typically should not exceed 1 μm from the radius of the tunnel contact. Further preferable maximum distances are 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 μm.

  FIG. 5 clearly shows the different conditions realized by the present invention compared to the prior art conditions shown in FIG. As a result of the structuring according to the present invention, the strong index guide present in FIG. 3 is changed to a weak index guide, the effective refractive index is averaged from the center to the outer region of the aperture.

  FIG. 6 shows another embodiment of a waveguide structure according to the present invention. In this configuration, the contact layer 8 is provided only as an option, and in this case, the opening diameter can be selectively structured in the same manner as described above with w3. The waveguiding properties are influenced in this case via an additional layer 9 which can be made from a freely selectable material. The additional layer 9 has an inner diameter w4 and an outer diameter w5, and is structured using, for example, an etching process or a lift-off method. The same principle is applied to the shape of the final thickness d4 and the inner diameter w4 as in the case of the contact layer as the structured layer in FIG. The advantage of this embodiment is that the material of the structured layer 9 can be freely selected. This can be used in particular to weaken the more extreme high modes that normally have a maximum at the edge of the aperture, thus preventing these modes from oscillating. Amorphous silicon is a suitable material for layer 9.

  Finally, FIG. 7 shows a third embodiment of a waveguide structure according to the invention with anti-guide action. In this case, the present embodiment is selected as the structured layer so that the thickness d3 of the contact layer 8 is sufficiently thicker than the depth d2 of the raised portion of the tunnel contact portion or the etching depth d1. This results in an increase in the external area and leads to an anti-guiding action, thus eliminating the high mode. When the layer 8 has an absorbing action, the mode removal is more effective. This embodiment shown in FIG. 7 can be combined with the structure shown in FIG. In this case, at least layer 9 includes the ridges shown.

  FIG. 8 shows a finished BTJ-VCSEL according to the present invention. A further process of the structure according to the invention for finishing a semiconductor laser has already been described in detail in connection with FIG. 2 in the introduction of the description. The same reference numbers refer to the same layers as the structure of FIG. In this case, the InP1 substrate is totally removed. Instead, for example, an n-side contact portion 14 made of Ti / Pt / Au is attached to the current supply means. The waveguide structure of the semiconductor laser shown in FIG. 8 corresponds to that of FIG. 5 by the contact layer 8 adjacent to the raised portion 15. 10 indicates an insulating passive layer, 11 indicates a p-side contact (for example, Ti / Pt / Au), 12 indicates a dielectric mirror, and 13 indicates an integrated heat sink.

  The present invention enables the manufacture of BTJ-VCSELs with high single mode power. The diameter of the aperture can be expanded to increase power without inducing high modes.

1 is a schematic diagram of the structure of a buried tunnel contact in a known surface emitting semiconductor laser. 1 is a schematic diagram of a completed structure of a surface emitting semiconductor laser, known from examples. 1 is a schematic diagram of conditions in a well-known structure of a surface emitting semiconductor laser associated with a contact layer and a current carrying layer. 1 shows a structure formed during the manufacture of a surface emitting semiconductor laser according to the invention. 1 is a schematic illustration of the structure of a contact layer as a structured layer according to the present invention. Figure 3 shows a further embodiment in which an additional layer is provided next to the contact layer as a structured layer according to the invention. Figure 6 shows a further embodiment with anti-guide action. Finally, an example of a surface emitting semiconductor laser manufactured according to the present invention is shown.

Claims (10)

  an active zone (4) having a pn junction and surrounded by a first n-doped semiconductor layer (3) and at least one p-doped semiconductor layer (5); and the p-side of the active zone (4) And a tunnel contact layer (6), the tunnel contact layer (6) having an opening having an opening diameter (w1) and an opening depth (d1), and n The adjacent current carrying layer (7) covered with the doping current carrying layer (7) has a raised portion (15) having a raised portion diameter (w2) and a raised portion depth (d2) in the region of the opening. And a structured layer (8; 9) is provided on the current carrying layer (7) at least around the side region of the raised portion (15), and the thickness (d3; d4) is the structured layer. In the region where the ridge depth (d2) is And a surface emitting semiconductor laser selected to be at least equal to the optical thickness of the current carrying layer (7).   2. The semiconductor laser according to claim 1, wherein an inner diameter (w3; w4) of the structured layer (8; 9) is greater than or equal to a diameter (w2) of the raised portion (15).   The half of the inner diameter (w3; w4) is at most 2 μm, preferably at most 1 μm, larger than the half of the diameter (w2) of the ridge (15). Or the semiconductor laser of 2.   4. The semiconductor laser according to claim 1, wherein the structured layer (8) is an n-doped contact layer (8).   The semiconductor laser according to claim 4, wherein the structured layer is made of n-doped InGaAs.   4. The semiconductor laser according to claim 1, wherein the structured layer (9) is made of a material having a considerable absorption action for each wavelength of the semiconductor laser.   7. A semiconductor laser according to claim 6, characterized in that amorphous silicon is used as the material, particularly in the wavelength range of 1.3 to 1.55 [mu] m.   The semiconductor laser according to claim 6, wherein titanium is used as the material.   9. An n-doping contact layer (8) surrounding the structured layer (9) from the side is provided next to the structured layer (9). The semiconductor laser as described in any one of them. 10. The thickness of the structured layer (8; 9) and / or the contact layer (8) is a multiple of the depth (d2) of the raised portion (15). The semiconductor laser described.